Theresa Penelope Raines was born in Eugene, Oregon, the youngest of three daughters of Clarence and Judy Raines. Her family was in the trucking business. During her childhood, her father constantly brought home injured animals from the highways on which his trucks travelled; this eventually instilled in her an ongoing commitment to saving and rehabilitating wild animals. While working in the family business in 1986, she started a rehabilitation facility called "Cougar Country" to re-educate and release predator mammals such as foxes, raccoons, bears, bobcats, and cougars back into the wild. Soon she was handling 300 animals each year.

Irwin joined an emergency veterinary hospital in 1989 as a veterinary technician to gain further knowledge on the care and support to all kinds of animals. Her life was very busy, as she was still helping her father run the family business, rehabilitating animals through her "Cougar Country", and working at the vet hospital. In addition, she had fifteen cats of her own, several birds and a dog.

In 1991, she went on a tour of Australia, and while visiting wildlife rehabilitation facilities, she had a chance meeting with Steve Irwin, whose father had founded the Australia Zoo (as Beerwah Reptile and Fauna Park). Steve would later say that "it was love at first sight." A "whirlwind romance" followed: They were engaged after only four months, and eight months later, on June 4, 1992, they married in Eugene, Oregon. Their first television documentary was filmed on their honeymoon. The footage, shot by John Stainton, became the first episode of The Crocodile Hunter,[2] which later became successful in the United States.

The couple settled in Australia shortly after their wedding, Terri leaving her Cougar Country project behind in the United States. However, as a partner in their wildlife enterprises and television shows, she believes she was able to do far greater work on behalf of wildlife conservation.

The Irwins had two children: daughter Bindi Sue on July 24, 1998, and son Robert Clarence on December 1, 2003.[4]

In an interview before the birth of their second child, Terri Irwin had this to say about her marriage and working with her husband Steve:

"We don't drink, we don't smoke, and we are actually in love and happily married. We love our little girl, we go home to each other at night, and we believe in what we are doing. Say my husband had a dangerous job and I wasn't with him; I don't know how you go, 'Oh honey, how was it with the police department today? You got all your fingers and toes today?' It would scare me. I'd have to become a police officer and work with him; I couldn't do it."[5]

In her first statement since her husband's death, Irwin announced the Australian memorial service (on September 20, 2006 at Australia Zoo in Queensland) would be open to the public, and that people who wished to attend should make a donation to Irwin's "Wildlife Warriors" fund. The service was held at the "Crocoseum," a 5,500-seat open-air amphitheater, which Steve built at the zoo and which she chose over any larger facility. She also thanked well-wishers for their "overwhelming outpouring of love, support and prayers for [her] family".[7] Terri herself was seemingly too upset to speak at the ceremony. She remained with her son, Bob, during the proceedings, but daughter Bindi spoke about her love for her father, for which she received a standing ovation.

Australian TV network Channel 9 screened an interview between Irwin and local presenter Ray Martin, on Wednesday September 27, at 8:30pm on the station. During the interview, she said "And I'll make Australia Zoo bigger. I'll make it bigger... because I promised." Barbara Walters held an interview with Irwin on The View, which aired September 27 in the US.

Terri with her daughter Bindi in September 2007.

On October 31, 2006 Irwin was invited to the Royal Albert Hall to award a Special Recognition Award to Sir David Attenborough at the British National Television Awards.[8] When she came on stage, the entire audience gave her a standing ovation. Irwin fought back tears, while the camera cut to Neighbours star Alan Fletcher, who was shown to be fighting back tears also. She cited Attenborough as a great inspiration for her husband, saying "If there's one person, other than his father, who directly inspired my husband, it's the person being honoured tonight." and going on to say "[Steve's] real, true love was conservation- and the influence of tonight's recipient in preserving the natural world has been immense."[8]

Attenborough reciprocated by praising her husband for introducing many to the natural world, saying "He taught them how wonderful and exciting it was; he was a born communicator."[9]

On January 3, 2007, the only video footage showing the events that led to Steve Irwin's death was handed over to Terri, who said the video would never become public, and noted her family has not seen the video either. In a January 11, 2007 interview with Access Hollywood, Terri said "all footage has been destroyed." On March 2, 2008 it was announced that Bob Irwin, father of Steve Irwin, had resigned from Australia Zoo, of which he was the founder, in order to "keep his son's dream alive" on a different property with his wife. In a statement to the press, the elder Irwin thanked the staff for all their help but made no mention of Terri. At the same time, the zoo was being sued for $2.5 million by a debt collection agency,[10] but the lawsuit was later dropped in an out-of-court settlement.[11]

In 2006, Irwin was made an honorary Member of the Order of Australia for services to wildlife conservation and the tourism industry.[14][15][16] ("Honorary" membership in the Order of Australia is a version of the award given to non-citizens of Australia; this became a substantive award when she became an Australian citizen in 2009.[17]) She has been awarded an honorary doctorate by the University of Queensland for her work in conservation and support of high-quality research.[18]

Irwin, American by birth, became an Australian citizen on November 15, 2009, in a tribute to her husband Steve. The citizenship ceremony was held during the Steve Irwin Day celebrations in Australia Zoo's Crocoseum.[1][20]